1. Field of the Invention
The present invention relates to an image signal processing apparatus for processing an analog output signal of an image sensor in a facsimile transmitter or an image scanner.
2. Description of the Related Art
In an image signal processing apparatus for an image sensor, there have been known various detection systems for detecting deterioration of the image sensor or the like.
In a first prior art detection system, an analog output signal of the image sensor obtained by scanning a white reference plate is supplied to an analog/digital (A/D) converter After an A/D conversion is carried out, a maximum value is detected by a maximum value detection circuit in an output signal of the A/D converter. Then, the maximum value is compared with a threshold value by a comparator. As a result, when the maximum value is smaller than the threshold value it is determined that a light source or the white reference plate is deteriorated, thus activating an alarm (see JP-A-63-1249).
In a second prior art detection system, also after an A/D conversion is performed upon the analog output signal of the image sensor it is determined whether or not a shading distortion of an effective line due to the deterioration of the light source or the like is smaller than a threshold value. As a result, if the shading distortion is not smaller than the threshold value, i.e., if the shading distortion cannot be corrected, a flip-flop is set so that this shading distortion on can be recognized (see JP-A-2-96474).
A third prior art detection system includes an additional monitoring light receiving element and an additional monitoring light receiving circuit which is connected to an image signal processing apparatus. In a monitoring state, the monitoring light receiving element and the monitoring light receiving circuit are operated, and it is determined whether or not the image signal processing apparatus is normally operated (see JP-A-2-90089).
A fourth prior art detection system provides a means for determining whether or not a document size detecting sensor is abnormal (see JP-A-2-132427).
In any of the above-described prior art detection systems, however, it is impossible to determine an amount of deterioration of the image sensor.